1. Field of the Invention
The present invention relates to a webbing tension device for imparting a slack-eliminating tension to a vehicle seat belt during an emergency situation of a vehicle in order to closely and safely restrain the occupants of the vehicle.
2. Description of the Prior Art
Seat belt systems for protecting vehicle occupants in emergency situations are known, wherein one end portion of the webbing is retracted in layers into a webbing retractor and fastened about the occupant so that the occupant is restrained by the webbing during an emergency situation of a vehicle.
The webbing retractor which retracts the webbing imparts a predetermined tension to the webbing, but the retracting force of the webbing retractor is either made relatively weak or non-existent in order to form a clearance or "slack " between the occupant and the webbing fastened thereabout, so as not to unduly squeeze the occupant during the normal operation of the vehicle.
However, in an emergency situation of a vehicle, such as a collision, the occupant is propelled in the direction of the collision by the clearance amount until finally restricted by the webbing.
To eliminate this slack in the webbing and the resulting injuries which may result therefrom in an emergency situation of a vehicle, a webbing tension device has been proposed in the prior art which transmits a biasing force from a spiral spring to a webbing takeup shaft located within the webbing retractor, thereby imparting a substantial tension to the webbing. In such a device, only a biasing force from a small spiral spring acts upon the takeup shaft in ordinary running of the vehicle, so that the tension of the webbing is normally kept relatively low. However, in an emergency situation of a vehicle, trigger means disposed in the webbing tension device couples a biasing force of a large spiral spring to the takeup shaft through a clutch means, thereby imparting a considerable retracting tension to the webbing. The clutch means of the webbing tension device includes an operation wheel for receiving the biasing force of the large spiral spring, and a gear wheel fixed to the takeup shaft. The operation wheel and gear wheel are connected to each other through a lever means actuated by an emergency situation of a vehicle, and the biasing force of the large spiral spring is imparted strikingly to the lever means. The problem with this device is that the clutch lever means must be manufactured heavily in order to be tough enough to withstand large forces, which in turn causes the clutch means to be made large-sized.